docs: def get_init_file(dir):
      # type: (str) -> Optional[str]
      """Check whether a directory contains a file named __init__.py[i].
      If so, return the file's name (with dir prefixed).  If not, return
      None.
      This prefers .pyi over .py (because of the ordering of PY_EXTENSIONS).
      """
      for ext in PY_EXTENSIONS:
          f = os.path.join(dir, '__init__' + ext)
          if os.path.isfile(f):
              return f
      return None